The present invention relates generally to gas turbine engines, and, more specifically, to vibration therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases discharged into a high pressure turbine (HPT) that powers the compressor. And, the gases are then discharged through a low pressure turbine (LPT) that typically powers a fan in a turbofan aircraft engine application.
The HPT rotor blades power the corresponding rotor blades of the compressor through one drive shaft, and the LPT rotor blades power the fan blades through another drive shaft. The drive shafts and associated rotor blades are suitably mounted in bearings supported in corresponding frames in the engine.
The HPT and its drive shaft operate at substantially high rotary speed during operation, and the LPT and its drive shaft operate at a relatively low rotary speed for powering the fan blades for maximum propulsion efficiency during aircraft flight.
The turbine engine is axisymmetrical about a longitudinal or axial centerline axis and is subject to small variations in dimensional configurations of its many components. Accordingly, the center of gravity of each rotary stage in the engine may be slightly offset or eccentric from the centerline axis which will then result in corresponding vibratory unbalance of the engine during operation over its operating speeds.
Accordingly, known procedures are used to measure rotor unbalance and then corrective balance weights may be installed in the engine to drive the center of gravity closer to the centerline axis and reduce undesirable levels of unbalance.
Since a gas turbine engine has many stages of rotor blades, engine balancing may be effected at any one or more of the various stages. However, in practice the inherent complexity of the modern gas turbine engine prevents unobstructed access to most of the rotating stages, and balance corrections are typically applied at an accessible stage such as the fan.
Although each rotor stage may contribute to the resulting engine unbalance, single-plane balancing of the entire engine is typically sufficient for reducing engine unbalance to a suitably small level.
In one conventional method of balancing a gas turbine engine, a vibration sensing accelerometer is installed in the engine at any suitable location for detecting vibrations or vibratory response of the engine. Since the rotor unbalance of engine is carried through the bearings which support the drive shafts in corresponding engine frames, the vibration sensor is typically located in one of the bearing planes in the corresponding supporting frame for detecting vibratory response.
The total vibratory response of the engine may therefore be measured in a single sensor which produces a single vibration signal containing the various frequencies of vibration in the engine.
Since the center of gravity unbalance of the rotor repeats once per revolution (1/rev) it provides a fundamental or synchronous vibration component of the total vibration signal, with higher order harmonics thereof. Furthermore, other vibratory response of the many rotating components of the engine are also detected by the sensor at corresponding vibratory frequencies.
Conventional procedures may then be used to filter from the total vibration signal its many constituent parts at corresponding natural frequencies. For example, a Fast Fourier Transform (FFT) is commonly used to transform the time domain of the total vibration signal into the frequency domain having corresponding amplitudes of the resonant responses.
Since the fan and compressor drive shafts operate at different rotary speeds the corresponding synchronous vibrations therefrom will occur at two distinct fundamental frequencies, and therefore provide a direct indication of corresponding vibratory unbalance of their corresponding rotors.
Since the relatively large fan typically introduces a correspondingly large component in engine vibration, the fan itself may be balanced using a corresponding balance weight for reducing the overall engine vibration levels.
The requisite balance correction may be mathematically computed in a conventional process using influence coefficients such as those described by Thomas P. Goodman in his paper in the Journal of Engineering for Industry, August 1964, pp. 273-279, and entitled “A Least-Squares Method for Computing Balance Corrections,” incorporated herein by reference.
In this balancing technique, a sample balance weight, or unit correction mass, is installed on a rotor and its affect on rotor vibration is measured. Since the balance weight is located at a specific radius from the centerline axis of the rotor, its affect is measured in mass and radius, and is typically expressed as ounce-inch, or g-cm. The resulting vibration of the rotor as detected by the vibration sensor will have a corresponding amplitude in mils, for example, at a corresponding synchronous frequency.
Accordingly, by knowing the influence coefficients for a particular rotor, then suitable corrective balance weights may be determined for offsetting measured vibratory response attributed to rotor unbalance.
Indeed, a vibratory monitor in the form of a dedicated computer is commonly used in modern turbofan aircraft engines for automatically monitoring in real time the vibratory response of the engine. The measured signal includes total vibration which is resolved in the frequency domain for synchronous vibrations. Since each engine is initially balanced at the factory, monitoring of rotor vibration may be used to detect changes in the engine which increase unbalance over engine life.
Fundamental to this vibration monitoring procedure and resulting balancing is the different rotary speeds of the drive shafts which result in different synchronous vibrations in the measured total vibration signal, which are readily distinguishable.
However, a new type of turbofan engine creates a special problem in vibration monitoring and subsequent balancing. In the unducted fan (UDF) turbofan engine, the LPT has a special configuration including interdigitated rotor blades supported on corresponding LPT rotors and from which corresponding rows of unducted fan blades extend radially outwardly.
The two LPT rotors are designed for counterrotation at equal but opposite rotary speeds and have the particular advantage of high propulsion efficiency for improving specific fuel consumption (SFC).
The engine includes a digital computer controller configured for matching the rotary speeds of the two rotors to reduce or prevent undesirable beating thereof which would introduce undesirable noise into the aircraft cabin.
Since the two rotors have matched operating speed, the measured vibratory responses thereof will occur at the same frequency and it is therefore impossible to distinguish the separate vibratory responses of the two rotors from each other using the conventional frequency analysis procedure described above.
Accordingly, it is desired to provide a new frequency analysis method for monitoring vibration in a dual rotor engine.